1. Field of the Invention
The present invention relates to a process for producing a laminated semiconductor substrate, and more particularly to a process for producing a laminated semiconductor substrate which is suitable for use in the production of a semiconductor-on-insulator (SOI) substrate through lamination.
2. Description of Related Art
An SOI substrate has hitherto been produced as follows (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 63-65648).
At the outset, as shown in FIG. 1A, an SiO.sub.2 film 2 is formed on a principal surface of a first silicon substrate 1 by thermal oxidation. Then, an SiO.sub.2 film 5 is formed on a principal surface of a second silicon substrate 3.
Then, as shown in FIG. 1B, the principal surface of the first silicon substrate 1 is laminated on the principal surface of the second silicon substrate 3, and the laminate was then heat-treated to bond the silicon substrates to each other.
Then, as shown in FIG. 1C, the second silicon substrate 3 is polished from the reverse side thereof to form an SOI substrate while leaving the SiO.sub.2 film 2 and the thin silicon film 3 as an active region.
In this process, however, it is difficult to monitor the degree of polishing in the step of final polishing, which makes it difficult to precisely control the thickness of the silicon film 3.
In order to solve this problem, the following means have been used.
At the outset, as shown in FIG. 2A, an SiO.sub.2 film 2 is formed on the whole principal surface of a first silicon substrate 1 by thermal oxidation. Separately, part of a second silicon substrate 3 is thermally oxidized to locally form a SiO.sub.2 film 4. The surface of the SiO.sub.2 film 4 is protruded from the surface of the principal surface of the silicon substrate 3, and the interface of the SiO.sub.2 film 4 and the silicon substrate 3 is located at a position deeper than the surface of the principal surface of the silicon substrate 3. The principal surface of the silicon substrate 3 is then oxidized.
Thereafter, as shown in FIG. 2B, the principal surface of the first silicon substrate 1 is laminated on the principal surface of the second silicon substrate 3, and the laminate is heat-treated to bond the substrates to each other.
Then, as shown in FIG. 2C, the second substrate 3 is polished from the reverse side thereof. In this case, since the polishing rate of the SiO.sub.2 is considerably smaller than that of the silicon, the SiO.sub.2 film 4 formed on the silicon substrate 3 serves as a stopper and the thickness of the silicon substrate 3 left as an active region on the SiO.sub.2 films 2 and 5 can be made constant.
In the process shown in FIGS. 2A to 2C, however, since protrusions of the SiO.sub.2 film 4 are present on the principal surface of the second silicon substrate 3, the lamination of the substrates to each other as shown in FIG. 2B causes voids to be formed around the protrusions, so that problems occur such that the bond deteriorates and the principal surface of the silicon substrate 3 around the protrusions cannot remain flat and thereby deflects to deteriorate the crystallinity of the silicon.
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a process for producing an SOI substrate that can control a silicon film to be served as an active region so as to have a constant thickness and enable an SOI substrate to be produced without detriment to the crystallinity and bonding property.